With the development of information industry, a large capacity storage device and data processing in high speed are required. Of devices for recording digital data, widely known holographic Read Only Memory (ROM) can store a large volume of information on each area corresponding to one bit of a storage medium, such as a Compact Disk (CD) and a Digital Versatile Disk (DVD), and has a high input and output speed because it processes data in parallel. Due to these advantages, the holographic ROM attracts attention as a future large capacity storage device.
Meanwhile, an optical disk system such as a typical CD or DVD system employs a tracking servo and a focusing servo to condense light into track-sized light and detect signal light reflected by a data track of an optical disk such as CD or DVD. More specifically, the tracking servo functions to control the position of condensed light in the radial direction of the optical disk to place the condensed light exactly within the data track of the optical disk, and the focusing servo controls the focal position of light in an optical axis direction to focus the light exactly on the reflection surface of the optical disk.
In contrast, a holographic ROM reader employing the holographic ROM irradiates reference light onto a disk shaped holographic storage medium. In this case, the size of the reference light is far larger than that of data track of the holographic storage medium being rotated, so that it is not necessary for the holographic ROM reader to exactly control the position of the reference light in the radial direction. That is, even though some run-out occurs in the radial direction during the rotation of the holographic storage medium, a desired data track is positioned within the irradiated reference light, so that the holographic ROM reader does not require the tracking servo as used in the optical disk system. In addition, the focusing depth of the reference light having a large diameter is large (for example, the focusing depth of 100 μm diameter light is about 4.7 mm). Accordingly, the data track of the storage medium is located within the focusing depth of the reference light even though some run-out occurs in the optical axis direction during the rotation of the storage medium, and therefore, the focusing servo is not required in the holographic ROM reader, unlike the optical disk system.
Therefore, the operation of the tracking and focusing servos for the holographic ROM reader is merely to pass signal light, which is reflected from the data pit of tracks on a storage medium, exactly through the pinhole of a pinhole plate, unlike the optical disk system. However, the size of the pinhole is same as that of the track pitch of the storage medium, so that it is difficult for the signal light formed by the data pit of a specific track to be incident exactly on the pinhole of the pinhole plate due to run-out in a radial direction or an optical axis direction that is caused by the rotation of the storage medium. Accordingly, the servo control for the holographic ROM reader should detect the signal light passed through the pinhole and control the signal light so that the signal light passes exactly through the pinhole based on detected result.
FIG. 1 is a schematic diagram showing a conventional holographic ROM reader. The conventional holographic ROM reader includes a reflection mirror 40 for allowing reference light, which is generated by a light source 10, to be incident on a storage medium 1 at a preset angle by reflecting the reference light, a pinhole plate 70 for passing only part of signal light having a diameter (corresponding to a one-bit data), which is diffracted from the storage medium 1, through the pinhole 75 thereof, and a Photo Detector Integrated Circuit (PDIC) 90 for detecting the signal light incident through the pinhole 75 in the pinhole plate 70.
The holographic ROM reader further includes a reducer 20 for transforming light between the light source 10 and the reflection mirror 40, and an aperture plate 30 for irradiating light, which is passed through the reducer 20, onto the reflection mirror 40 through the aperture thereof. In addition, an objective lens 60 is placed between the storage medium 1 and the pinhole plate 70 to condense the signal light from the storage medium into the pinhole 75, and a condenser lens 80 is provided between the pinhole plate 70 and the PDIC 90 to focus the light passed through the pinhole 75 in the pinhole plate 70.
The operation of the conventional holographic ROM reader will be described as follows. Laser light generated by the light source 10 is incident on the reflection mirror 40 through the reducer 20 and the aperture plate 30. The reflecting mirror 40 reflects the reference light, which corresponds to the phase-conjugate wave of reference light used at the time of recording data on the storage medium 1, onto the storage medium 1 at an incident angle identical to that at which light was incident on the storage medium 1.
The reference light incident on the storage medium 1 is diffracted through a recording material layer of the storage medium 1 to be signal light. The diffracted signal light is then focused on the pinhole plate 70 by the objective lens 60. In this case, the diameter of the reference light incident on the storage medium 1 is about 100 μm, so that the signal light, which is diffracted from the storage medium 1 and is focused through the objective lens 60, carries data reproduced from several tracks of the storage medium 1. The signal light is passed through the pinhole 75 of the pinhole plate 70, is condensed by the condenser lens 80, and is then transmitted to the PDIC 90. In this case, the pinhole 75 in the pinhole plate 70 has a size corresponding to the track pitch of the storage medium 1, that is, a one-bit data size, so that only part of the signal light (corresponding to the data bit of a track) is passed through the pinhole 75 and is condensed onto the PDIC 90 through the condenser lens 80.
The PDIC 90 detects the amount of the signal light. However, the conventional holographic ROM reader is problematic in that it is difficult to precisely determine whether the signal light has passed through the center of the pinhole 75 in the pinhole plate 70 or through the side of the pinhole 75, based on the amount of light measured by the PDIC 90. Accordingly, the conventional holographic ROM reader cannot perform a precise control to allow the signal light to pass through the center of the pinhole in the pinhole plate.